diff --git a/src/aws_durable_execution_sdk_python_testing/executor.py b/src/aws_durable_execution_sdk_python_testing/executor.py
index 2b2fbe7..02a1504 100644
--- a/src/aws_durable_execution_sdk_python_testing/executor.py
+++ b/src/aws_durable_execution_sdk_python_testing/executor.py
@@ -323,14 +323,14 @@ def stop_execution(
 
         Raises:
             ResourceNotFoundException: If execution does not exist
-            ExecutionAlreadyStartedException: If execution is already completed
         """
         execution = self.get_execution(execution_arn)
 
         if execution.is_complete:
-            # Context-aware mapping: execution already completed maps to ExecutionAlreadyStartedException
-            msg: str = f"Execution {execution_arn} is already completed"
-            raise ExecutionAlreadyStartedException(msg, execution_arn)
+            # Idempotent: return the existing stop timestamp
+            execution_op = execution.get_operation_execution_started()
+            stop_timestamp = execution_op.end_timestamp or datetime.now(UTC)
+            return StopDurableExecutionResponse(stop_timestamp=stop_timestamp)
 
         # Use provided error or create a default one
         stop_error = error or ErrorObject.from_message(
diff --git a/tests/executor_test.py b/tests/executor_test.py
index 8e50f2a..e228a0d 100644
--- a/tests/executor_test.py
+++ b/tests/executor_test.py
@@ -41,6 +41,7 @@
     SendDurableExecutionCallbackHeartbeatResponse,
     SendDurableExecutionCallbackSuccessResponse,
     StartDurableExecutionInput,
+    StopDurableExecutionResponse,
 )
 from aws_durable_execution_sdk_python_testing.observer import (
     ExecutionNotifier,
@@ -2056,13 +2057,22 @@ def test_stop_execution(executor, mock_store):
 
 
 def test_stop_execution_already_complete(executor, mock_store):
-    """Test stop_execution with already completed execution."""
+    """Test stop_execution with already completed execution returns idempotent response."""
     mock_execution = Mock()
     mock_execution.is_complete = True
+    mock_execution.durable_execution_arn = "test-arn"
+
+    # Mock the execution operation with end_timestamp
+    mock_execution_op = Mock()
+    mock_execution_op.end_timestamp = datetime(2023, 1, 1, 0, 1, 0, tzinfo=UTC)
+    mock_execution.get_operation_execution_started.return_value = mock_execution_op
+
     mock_store.load.return_value = mock_execution
 
-    with pytest.raises(ExecutionAlreadyStartedException, match="already completed"):
-        executor.stop_execution("test-arn")
+    result = executor.stop_execution("test-arn")
+
+    assert isinstance(result, StopDurableExecutionResponse)
+    assert result.stop_timestamp == datetime(2023, 1, 1, 0, 1, 0, tzinfo=UTC)
 
 
 def test_stop_execution_with_custom_error(executor, mock_store):
diff --git a/tests/web/handlers_test.py b/tests/web/handlers_test.py
index 167b0f5..d08932b 100644
--- a/tests/web/handlers_test.py
+++ b/tests/web/handlers_test.py
@@ -911,14 +911,15 @@ def test_stop_durable_execution_handler_success():
 
 
 def test_stop_durable_execution_handler_execution_already_stopped():
-    """Test StopDurableExecutionHandler with execution already stopped error."""
+    """Test StopDurableExecutionHandler with execution already stopped returns idempotent response."""
 
     executor = Mock()
     handler = StopDurableExecutionHandler(executor)
 
-    # Mock executor to raise IllegalStateException
-    executor.stop_execution.side_effect = IllegalStateException(
-        "Execution test-arn is already completed"
+    # Mock executor to return stop response with timestamp
+    stop_timestamp = "2023-01-01T00:01:00Z"
+    executor.stop_execution.return_value = StopDurableExecutionResponse(
+        stop_timestamp=stop_timestamp
     )
 
     request_body = {
@@ -941,10 +942,9 @@ def test_stop_durable_execution_handler_execution_already_stopped():
 
     response = handler.handle(typed_route, request)
 
-    # Verify IllegalStateException maps to ServiceException in AWS-compliant format
-    assert response.status_code == 500
-    assert response.body["Type"] == "ServiceException"
-    assert response.body["Message"] == "Execution test-arn is already completed"
+    # Verify idempotent response with stop timestamp
+    assert response.status_code == 200
+    assert response.body["StopTimestamp"] == stop_timestamp
 
 
 def test_stop_durable_execution_handler_resource_not_found():